Electrophoretic insulating coating



Filed April 9, 1958 3 m F fin 9 5 is M V w J 9 7 7 5 m a n w M p 5 K I 3 u O N. o flflmmn \Q. a T 3 3 a 3 u I 0. WP 0 9 g H PS 2 ,F Hm

mvamon James K. Thomson flflw w ATTORNEY will than": Patented Oct. 18, 1960 ELECTROPHORETIC INSULATING COATING James K. Thomson, Bath, N.Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa, 21 corporation of Pennsylvania FiledApnQ, 1958, Ser. No. 727,314

5 Claims. (Cl. 204-181) This invention relates to an insulating coating and, more particularly, to an insulating coating deposited on a conductive member by electrophoretic methods. While this invention may be utilized in the coating of any conductive member, it will be disclosed with particular emphasis on the coating of heater filament members for use in electric devices.

There are many methods of applying insulating coatings to conductive bases. The method used depends on the ultimate use of the coated member, the cost, the material to be coated, etc. The. methods used most in the electronic tube industry are a drag process, a spraying process or an electrophoretic process. In the latter process, charged particles of a coating material are directed to the conductive base when brought under the influence of an electric field.

It has been found that for use in electronic devices the usual drag coating methods are adequate in certain respects but usually provide a rough, non-concentric coating and require a large number of steps during preparation. For example, in the drag coating process a wire is dragged through a suspension of an aluminum oxide and aluminum nitrate. It is found that a small amount of material is coated on the wire. In order to make this coating of sufiicient thickness, the wire must be passed through the coating material a number of times. After each coating operation, the wire must be baked in an oven, usually in an air atmosphere, to insure adequate adherence. After the proper diameter is obtained, the coated wire is then passed through a high temperature hydrogen furnace to sinter the coating to reduce the oxide formed on the wire by the air oven, and to decompose the aluminum nitrate to aluminum oxide. However, even at best the coating formed is quite rough and has an egg-shaped cross section.

It has been found for certain uses that an electrophoretic method has advantages over the spray and drag coating methods because it provides a uniform concentric coating, eliminates filament oxidation, gives an improved coating smoothness and uniformity, is done by a comparatively simple process and is easily controlled. However, major disadvantages of this method, which have prevented its universal adoption, are the low electrophoretic properties and precipitation efiiciency of the coating suspension materials used. For example, one method of electrophoretic coating utilizes a suspension of aluminum oxide and aluminum nitrate, usually with an organic binder material, but, in the case of one particular wire for example,,the coating process cannot be done in less than from 6 to 12 seconds. Also comparatively high voltages are required.

I have found that by the use of a coating suspension of a novel composition that the above-mentioned wire may be coated rapidly (in about /2 second) and at low voltages (for example from 6 to volts).

Accordingly, it isan object of this invention to provide an improved insulating coating for a conductive member.

It is another object to provide an improved insulating coating for a heater filament.

It is a further object to provide an improved smooth, concentric, insulating coating for a heater filament.

It is a further object to provide an improved insulating coating for a. heater filament suitable for use in electronic devices.

It is an additional object to provide an improved insulating coating suspension for use in an electrophoretic deposition process..

It isv an auxiliary object to provide an improved method of applying an insulating coating for use in an electrophoretic deposition process.

These and other objects of this invention will be apparent from the following description, taken in accordance with the accompanying drawing, throughout which like reference characters indicate like parts, which drawing forms a part of this application and in which:

Figure 1 is a schematic drawing of a method and apparatus for applying an insulating coating to a conductive wire in accordance with one embodiment of this invention;

Fig. 2 is a schematic view of a method and apparatus for applying an insulating coating to a conductive Wire member in accordance with another embodiment of my invention; and

Fig. 3 is a sectional view taken along lines III-III of the coating trough 5L shown in Fig. 2.

In this application, including the claims, the word suspension includes, but is not limited to, colloids, suspensions, emulsions, sols and suspended particles in a generally wide range of sizes.

It is known that small particles, such as colloidal particles, will migrate under the influence of an applied electromotive. force and this phenomenon is known as electrophoresis. Generally, it is believed that a so-called electrical double layer is formed at the interface between the disperse phase and the dispersion medium with the consequent. presence of an electrokinetic potential between the fixed and freely mobile portions of the double layer. This electrokinetic potential is known as the zetapotential. When an electromotive force is applied, the positive and negative-portions of the double layer are displaced relative to each other, and they will, therefore, migrate in the electrical field. The velocity of the particles depend upon the zeta-potential (which in turn is determined by the nature and condition of the surface of the particles), the dielectric constant of the medium, the potential gradient and the coefficient of viscosity of the suspending medium. Sometimes when the particles of the suspension are positively charged and move toward the cathode under the influence of the applied electromotive force, the process is known as cataphoresis.

It is desirable to coat filaments with an insulating material such as aluminum oxide. It has been found that when aluminum oxide is in a suspension in Water with nothing else it tends to flocculate and deposits on the bottom of the container and therefore is unaffected by the passage of a current between two electrodes. By adding aluminum nitrate to the aluminum oxide suspension in water, a coating can be formed electrophoretically but it requires the application of comparatively high voltages (220 volts for example) and also takes a considerable period of time depending on the item to be coated. I have found that by forming a suspension of aluminum oxide with basic aluminum acetate in a polar solvent such as acetone that an electrophoretic deposition takes place very rapidly (on the order of /2 second) and requires only from 6 to 10 volts to continuously coat heater wire.

As referred to herein and in the appended claims the term polar solvent is used to designate a member of the group of substances known as polar solvents. In Concise Chemical and Technical Dictionary, H. Bennett, Editor, Chemical Publishing C0,, Inc., Brooklyn, N.Y., 1947, the term polar solvents is defined as solvents containing hydroxyl or carbonyl groups, having high dielectric constants and strong polarity, e.g. alcohols and ketones.

In one specific example of my invention I have found that if about 21 grams of basic aluminum acetate [Al(OH) (C O H is dissolved in about 105 cubic centimeters of deionized water, which is then added to about 2800 cubic centimeters of acetone followed by about 1250 grams of aluminum oxide, a much improved insulating coating suspension is formed. After milling this suspension for about 1 hour or longer, it may be applied by a method such as that shown in Fig. 1. In Fig. 1 there is shown a first spool 13 upon which a sufficient quantity of a suitable wire 11 is wound. This wire may be of any suitable electrode material, but in this particular application, I have found that tungsten and molybdenum-tungsten alloys are suitable. The wire 11 then extends around a first pulley 15 and down into a tank 25. This tank 25 is made of a suitable insulating material such as a borosilicate glass, containing approximately 58% silicate, 19% aluminum oxide, 12% magnesium, and boron oxide and small amounts of calcium and alkali, that is known under the Corning Glass Company trademark of Pyrex 1720. The wire extends around a second pulley 17 which is located within the tank 25 and then goes vertically through an electrode 27. The wire 11 leaves the tank and goes through a drying oven 37, around a third pulley 19, and through a high temperature furnace 39. The wire 11 then goes around a fourth pulley 21 and is finally wound on a second spool 23. As can be seen an inlet 31 enters the bottom of the tank 25 and an outlet 33 extends from near the top of the tank 25 to the bottom of the tank 25 and thence to a pump which is not shown. The pump forces the coating suspension 73 into the tank 25 and the suspension level 35 of the coating suspension is kept constant by the outlet 33. A suitable power supply 29 is electrically connected to the wire 11 on the first spool 13 and is also electrically connected to the electrode 27. Suitable materials for the electrode 27, which may be of cylindrical shape, include stainless steel, aluminum and molybdenum. T o avoid contamination because of abrasion the bearing of the second pulley 17 may be made of a suitable resistant material such as a tetrafiuoroethylene polymer known under the trademark of Teflon." I have found that if the wire goes through the coating tank 25 at a rate of approximately 37.2 feet per minute, at a suitable voltage of 8 volts, each portion of the wire 11 is immersed in the tank for approximately 1.6 seconds.

After leaving the tank 25, the coated wire goes through a drying oven 37 which is maintained at a temperature of approximately 60 degrees centigrade and then goes through a high temperature furnace 39. A suitable operating temperature for this furnace 39 is from about 1300 degrees centigrade to about 1700 degrees centigrade. The atmosphere in the furnace should not be injurious to the metallic base or the coating material. One particular atmosphere I have used is that comprised of 25% nitrogen and 75% hydrogen, but other compositions, of course, may be utilized. The purpose of heating the furnace 39 to a high temperature is to remove undesirable materials, to sinter the aluminum oxide coating and to reduce the basic aluminum acetate to aluminum oxide.

In some instances it may be desirable to utilize a system such as that shown in Figs. 2 and 3. This system has the advantages that it is simpler, it does not require the use of a drying oven, a coating tank, or as large a number of pulleys. In operation a first spool 43 has a sutficient amount of wire 41 wound upon it. The wire 41 extends around a first pulley 45 and through a coating trough 51 which will be described in detail later. The wire 41 then extends through furnace 71 similar to that shown in Fig. 1, around a second pulley 47 and is finally wound on a second spool 49.

The coating trough 51, a sectional view of which along lines IIl-III is shown in Fig. 3, includes an insulating liner 53 which may be made of a suitable material such as the previously mentioned Teflon. The coating trough 51 also includes a conductive electrode 55 which may be made of a suitable material such as aluminum and an insulating casing 57 around the electrode 55. The insulating casing 57 may also be made of a suitable material such as the Teflon mentioned above. The insulating liner 53 should be provided with holes 79 extending through the liner 53 to the conductive electrode 55.

An inlet 59, which may be made of a suitable material such as stainless steel, brings the coating suspension 75 from the pump (not shown) into the coating trough 51. The electrode 55 is electrically connected through the inlet 59 to a suitable power supply 77 which is also electrically connected to the wire 41 in the first spool 43. The wire 41 passes through the coating trough 51 at an approximate speed of 37.2 feet per minute and a voltage of approximately 50 volts is applied. Each portion of the wire 41 remains in the coating trough 51 at a period of approximately 0.3 second. The suspension overflow 65 falls from the ends 63 of the coating trough 51 and falls into the drain basin 69 (made of an insulating material such as Tefion) from which an outlet 61 returns the suspension material to the pump. It may be desirable in some instances to lower the coating trough 51 into the drain basin 69 in such a manner that the level of the suspension in the coating trough 51 is the same as the level of the coating suspension in the drain basin 69. It should be noted that the coating trough 51 is in a horizontal position which eliminates the necessity for passing the wire through the drying oven. The coated wire 41 may go directly to the furnace which is similar to that discussed in connection in Fig. 1 and operates in a similar manner. The fewer pulleys that the wire passes over the less chance of damaging the wire. Of course, it should be understood that other arrangements may be utilized in applying this insulating material electrophoretically.

I have found that suitable coating suspensions may be made if 1250 units of insulating oxide are used either from 8 to 40 units of basic aluminum acetate with from about 50 to 150 units of distilled or deionized water and from approximately 1000 to 5000 units of acetone or other suitable polar solvent, wherein a unit is a gram or a cubic centimeter depending on whether the constituent is a solid or liquid. The amounts of basic aluminum acetate and the contentration of aluminum oxide will vary according to the type of coating desired. Different types of coating may be desirable when heater filaments of different configurations are used and when it is desired to operate the heater filaments at different temperatures or in different devices. Also the applied voltage may vary between 2 and volts. I have found that coated heaters formed by this method are as good with respect to leakage and arcing as those done by other methods and gives finished electronic devices comparable to that obtained by other methods. Also the initial manufacturing shrinkage is comparable and in sertion rate of the heater into the cathode is higher. The advantage of this suspension over prior art suspensions is that it is many times faster and that it requires only low voltages which are safe to use and therefore is suit able to be used commercially.

Also if desired, titanium dioxide may be used in place of aluminum oxide in this invention.

It is, of course, obvious that this method may be utilized any place in which it is desired to have an insulating coating on a base member. While the base member must be conductive during the application of the coating, it is, of course, possible to apply the coating on a non-conductive base by the expedient of coating the base with a conducting layer which may be removed during or after the processing.

While the present invention has been shown in a few forms only, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various changes and modifications without departing from the spirit and scope thereof.

I claim as my invention:

1. An insulating coating suspension for electrophoretic deposition upon a conductive base, said coating suspension consisting essentially of a quantity of insulating oxide particles selected from the group consisting of aluminum oxide and titanium dioxide and, for each 1250 grams of said insulating oxide particles, from about 8 grams to about 40 grams of basic aluminum acetate, from about 50 cubic centimeters to about 150 cubic centimeters of water and from about 1000 cubic centimeters to about 5000 cubic centimeters of a polar solvent.

2. An insulating coating suspension for electrophoretic deposition upon a conductive base, said coating suspension consisting essentially of a quantity of insulating oxide particles selected from the group consisting of aluminum oxide and titanium dioxide and for each 1250 grams of said insulating oxide particles, from about 50 cubic centimeters to about 150 cubic centimeters of water, from about 1000 cubic centimeters to about 5000 cubic centimeters of acetone and from about 8 grams to about 40 grams of basic aluminum acetate.

3. The method of applying an insulating coating to a conductive base by electrophoresis, said method including the steps of forming a suspension consisting essentially of a quantity of an insulating oxide selected from the group consisting of aluminum oxide and titanium dioxide and, for each 1250 grams of said insulating oxide, from about 8 grams to about 40 grams of basic aluminum acetate, from about 50 cubic centimeters to about 150 cubic centimeters of water and from about 1000 cubic centimeters to about 5000 cubic centimeters of a polar solvent, positioning said conductive base in said suspension and electrophoretically depositing the solids from said suspension under an electric potential to form a coating on said conductive base.

4. The method of applying an insulating coating to a conductive base by electrophoresis, said method including the steps of forming a suspension consisting essentially of a quantity of an insulating oxide selected from the group consisting of aluminum oxide and titanium dioxide and, for each 1250 grams of said insulating oxide, from about 8 grams to about 40 grams of basic aluminum acetate, from about cubic centimeters to about cubic centimeters of water and from about 1000 cubic centimeters to about 5000 cubic centimeters of a polar solvent, positioning said conductive base in said suspension, electrophoretically depositing the solids from said suspension under an electric potential to form a coating on said conductive base, and heating said coating to increase the adherence of said coating to said conductive base.

5. The method of applying an insulating coating to a conductive base by electrophoresis, said method including the steps of forming a suspension consisting essentially of a quantity of insulating oxide selected from the group consisting of aluminum oxide and titanium oxide and, for each 1250 grams of said insulating oxide, from about 8 grams to about 40 grams of basic aluminum acetate, from about 50 cubic centimeters to about 150 cubic centimeters of water and from about 1000 cubic centimeters to about 5000 cubic centimeters of acetone, positioning said conductive base in said suspension and electrophoretically depositing the solids from said suspension under an electric potential to form a coating on said conductive base.

References Cited in the file of this patent UNITED STATES PATENTS 2,442,863 Schneider June 8, 1948 2,495,630 Dorst Jan. 24, 1950 2,530,546 Snyder Nov. 21, 1950' FOREIGN PATENTS 587,039 Great Britain Apr. 11, 1947 917,744 France Sept. 23, 1946 

1. AN INSULATING COATING SUSPENSION FOR ELECTROPHORETIC DEPOSITION UPON A CONDUCTIVE BASE, SAID COATING SUSPENSION CONSISTING ESSENTIALLY OF A QUANTITY OF INSULATING OXIDE PARTICLES SELECTED FROM THE GROUP CONSISTING OF ALUMINUM OXIDE AND TITANIUM DIOXIDE AND, FOR EACH 1250 GRAMS OF SAID INSULATING OXIDE PARTICLES, FROM ABOUT 8 GRAMS TO ABOUT 40 GRAMS OF BASIC ALUMINUM ACETATE, FROM ABOUT 50 CUBIC CENTIMETERS TO ABOUT 150 CUBIC CENTIMETERS OF WATER AND FROM ABOUT 1000 CUBIC CENTIMETERS TO ABOUT 5000 CUBIC CENTIMETERS OF A POLAR SOLVENT. 